This much is said of him in private by people who know him well, that he was one of the very few people who had given very serious thought over a period of time to how Brexit could work in advantageous terms as a practical endeavour. No stranger to Flexcit
, to which he constantly made reference, he became very scathing about the delusions of some Tory eurosceptics and of the blinkered slow-wittedness of the politicians he served.
By the beginning of 2016, as the contours of Mr Cameron's ill-fated new settlement between Britain and the EU had emerged, both Sir Ivan and Mr Scholar had already developed fairly advanced views on what Brexit might mean in practice.
Thus, despite Mr Cameron's injunction that there was to be no contingency planning, as Theresa May came to power, there was an embryonic plan in place, leaving Sir Ivan as the natural person to become Britain's "Mr Brexit".
Like other negotiators on both the British and EU sides, he has been dismissive of talk of "hard" or "soft" Brexit, telling London and Brussels that the real challenge is a "managed and orderly departure compared to a disorderly, chaotic exit" – straight out of the Flexcit playbook.
And now, with just weeks to go before Mrs May has to present her case in Brussels, Sir Ivan has resigned
- an unexpected event which leaves Whitehall without its most senior man in Brussels and a man slated as one of the very few at high levels in the Civil Service who really understands how the EU works.
Currently, it is being said that he has been "hounded out" by hostile pro-Brexiteers, a charge which has both detractors and supporters speculating as to proximate cause of his departure. But, by all, it is acknowledged that his expertise in Brussels will be very difficult to replace at such short notice.
From the look of it, though, he has been completely misread by the Brexit zealots
, who are applauding his premature departure, and calling for an end to "pessimistic mandarins".
But it is the Irish Times
which is prominently suggesting that a "hard" Brexit is now more likely. And an Irish official who had regular contacts with Sir Ivan expressed a lack of surprise at his departure, saying: "He may realise what an awful job it's going to be".
It that is worrying for Ireland, it is doubly worrying for those in the UK who are looking for some rationality in the debate, more so as his departure may be taken as a signal in Brussels that Whitehall is firming up for that "hard" Brexit.
However, with his tenure due to end in November, Sir Ivan himself writes that
it would make no sense to have a change-over when the negotiations had started. He says, therefore, that he has decided to step down now, "having done everything that I could in the last six months to contribute my experience, expertise and address book to get the new team at political and official level under way".
Nevertheless, in a barb that has not gone unnoticed, he writes to his former colleagues: "I hope you will continue to challenge ill-founded arguments and muddled thinking and that you will never be afraid to speak the truth to those in power". Senior ministers, he adds, "also need from you detailed, unvarnished - even where this is uncomfortable - and nuanced understanding of the views, interests and incentives of the other 27".
Thus does Sir Ivan appear voluntarily to have stepped away from an historical event, not even allowing for the possibility that his tenure might have been extended. But if his departure is shown to have had the fingerprints of No 10, it will give a considerable boost to David Davis, who is known to be intolerant of realistic appraisals of Brexit prospects, and makes a point of excluding critical advice.
In general, it is perceptions as much as the reality that counts. If the "colleagues" are expecting the worst and are already on the defensive, this departure will intensify their fears - however much it is said to be voluntary. With no-one on the spot to soothe their fevered brows, Mrs May can expect a rough ride when she gets to Brussels, whatever her actual intentions.
And for all that, perhaps the most worrying thing of all is the very suggestion that there are few people in high places who really understand how the EU works. If that is truly the case, and one of those people has just been forced out of office, we are in far more serious trouble than we can possibly imagine.